ARC VT100 with KT-120 tubes ?

Hi All. Did talk to Chris O. this week as well about dropping those KT120 into a pair of Classic 120. His response was that ARC have not "tested" those in a pair of 120's and will not until they get a pair in for service. He told me that electronically speaking there should be no issues using KT120 in the 120's except that there may not be enough range on the pot to adjust the bias up or down. A resistor may have to be changed. Physically speaking, they may not fit within the cage enclosure as they are taller. However, i operate my 120's with the cages off so that is not an issue. He said to me just drop them in and report back to us. Otherwise, it may be a while before they are tested in my 120's and the many, many other amps ARC has designed in the last 40 years. I am not quite willing to send my amps down to the Factory from Canada just to see if they are a fit or not. However, Chris appears to be very confident that they should work just fine in those 120's.
Long story short, i have to make a decision soon as i have 2000 hrs on my SED 6550C now.
Sly

I think they will work great and given the amp runs the outputs in triode it should sound fantastic!!!

Bifwyinne:
Thanks for being a good sport!

More plate voltage is not very important in the context of most tube amps, because the plate voltage is fixed by design. However, the ability to take more plate current is an advantage, for the following reasons:

The Svetlana 6550C has the following limiting values (only ONE limit can be exceeded at a time, and only momentarily, for example, during power up):
-680V plate voltage
-plate dissipation: 35 Watts as triode, 42 Watts as tetrode

For best sound / least distortion, ARC tends to run their tubes at about 450 Volts B+ and 65 ma current, which is aproximately 30 Watts, pretty close to the tube's limiting value.

The KT120 has the following limiting values:
-650 plate voltage connected as a triode, 850V as a tetrode
-Plate dissipation: 60 Watts

Running a 60W tube at 30W dissipation is much easier on the tube that running a 35-42W tube at 30W dissipation.

So, instead of plate voltage, power dissipation is the main KT-120 advantage.
A 60W tube running at 30 W dissipation will be loafing along and will probably last much longer than any 6550 tube, provided that the power transformer can take the extra heater current and that ARC makes the (depending on amplifier model) necessary bias supply changes.

I agree with you, this tube has great potential but is not a simple drop-in replacement.
Enjoy the music!

Casouza there's more to it. The construction of the KT-120 is far more robust than the 6550. Taller plate with heat sink fin structure to allow for more efficient heat dissipation, as well as a thicker glass envelope. The KT-120 also has getter flashing on the top and on the side. I do agree with you, it is NOT a drop in replacement.

Thansk Hifigeei and Casouza for the tech information. Hifigeek, care to share what's wrong with the VS115 you're working on??